1. Field of the Invention
The present invention relates to a force detection device used in a power-assisted truck or the like in which human power is assisted with motive power.
2. Description of Related Art
There has been conventionally proposed a truck having a power assist function for assisting human power with motive power. FIG. 18 shows a handle 7 of a conventional power-assisted truck. A pair of splicing fittings 73, 73 each having a load cell 71 attached thereto are removably provided at both ends of a hand push handle 74. Both splicing fittings 73, 73 are fitted to both ends of a U-shaped bar handle 72. When an operator applies a force to the bar handle 72, which is detected by two load cells 71, 71, torque to be given to a motor (not shown) is controlled based on a detection result of both load cells 71, 71 to realize a power assist function (see JP 6-304204 A).
When the bar handle 72 is given a force in a forward/backward direction of the truck, this force can be detected by two load cells 71, 71. However, when the bar handle 72 is given a force in a lateral direction orthogonal to the forward/backward direction, this force cannot be detected by two load cells 71, 71 because-two load cells 71, 71 are fitted to both ends of the U-shaped bar handle 72. The above conventional handle 7 has been having this problem.
A force detection device as shown in FIG. 19(a) has been proposed in order to solve this problem (see JP 3170438 B). A handle 81 is C-shaped. Both ends of the handle 81 are coupled to a truck body 84 via a pair of plate springs 82, 82. Facing each plate spring 82, a sensor 83 for detecting displacement of the plate spring 82 is provided at the truck body 84 side.
As shown in FIG. 19(a), when a force F1 in the backward direction of the truck body 84 is applied to the middle of the handle 81, a force in the pulling direction is applied to two plate springs 82, 82 to thereby flex two plate springs 82, 82 in the same direction.
On the other hand, as shown in FIG. 19(b), when a leftward force F2 as illustrated is applied to the middle of the handle 81, torque acts on the handle 81 to turn the handle 81, so that of two plate springs 82, 82, the left plate spring 82 is given a force in the pushing direction, while the right plate spring 82 is given a force in the pulling direction. This causes two plate springs 82, 82 to flex in opposite directions.
According to the above force detection device, because the handle 81 is coupled to the truck body 84 via the pair of plate springs 82, 82, not only a force in a forward/backward direction of the truck body 84 applied to the handle 81, but also a clockwise/counterclockwise turning force can be detected based on displacement of two plate springs 82, 82 detected by the pair of sensors 83, 83, an elastic coefficient of both plate springs 82, 82, a shape of the handle 81 and the like.
However, according to the above force detection device, if an operator grips the handle 81 at a position laterally shifted from the middle thereof and applies the forward/backward force, a rotational force acts on the handle 81 to displace two plate springs 82, 82 in opposite directions. In this case, a clockwise/counterclockwise turning force is supposed to have been applied to the handle 81 because an external force applied to the handle 81 is detected based on displacement of two plate springs 82, 82. Consequently, there has been a problem of the truck body 84 turning either clockwise or counterclockwise against the operator's intention of moving the truck body 84 forward or backward.
As a method for solving this problem, a method is possible of using a known position sensor or the like to detect a position where the operator grips the handle 81, and detecting an external force applied to the handle 81 based on a detection result of the gripped position and displacement of two plate springs 82, 82 detected by the pair of sensors 83, 83. However, if the operator grips the handle 81 with both hands but applies a force with only one hand, it is impossible to determine with which hand the force is applied even if such a method is used. Even if a force point where the external force is applied could be accurately detected, it would be necessary to expand a detection range of both sensors 83, 83 in order to detect a force applied to both ends of the handle 81. This has been causing a problem of a poorer detection resolution for a force applied to the middle of the handle 81.